It is desirable to reduce harmful emissions from diesel engine exhaust, to minimise the impact on the environment, and to comply with increasingly stringent emission regulations.
Many modern diesel engines employ exhaust gas re-circulation (EGR) for reducing harmful NOx emissions. However, there is a tendency for soot emissions to increase when high levels of EGR are employed. It is known that soot emissions from diesel engine exhaust can be reduced by providing high rates of air/fuel mixing to reduce the residence time of local fuel-rich mixtures within the combustion chamber.
A significant amount of research has been conducted into diesel engine exhaust emissions (Greeves G, Tullis S and Barker B, “Advanced Two-Actuator EUI and Emissions Reduction for Heavy-Duty Diesel Engines”, SAE 2003-01-0698; Tullis S and Greeves G, “HSDI Emission Reduction with Common Rail FIE”, S492/S18/99). This research has included optical studies of spray/jet formation from fuel injector nozzles (Browne K R, Partridge I M and Greeves G, “Fuel property effects on air/fuel mixing in an experimental diesel engine”, SAE 970185), computer models of fuel spray/jets (Partridge I M and Greeves G, “Interpreting Diesel Combustion with a Fuel Spray Computer Model”, I Mech. Eng., 1998), and experiments into combustion and soot formation (Khan I M, Greeves G and Probert D M, “Prediction of Soot and Nitric Oxide Concentrations in Diesel Exhaust”, I. Mech. E., 1971).
Modern diesel engines often utilise direct injection diesel combustion systems comprising multi-hole fuel injector nozzles. From the studies described above, it is concluded that the rate of air/fuel mixing, and hence reduction of soot in the engine exhaust, is significantly affected by the magnitude of the axial spray/jet momentum from the injector nozzle of each injected fuel spray/jet, rather than the fuel droplet size. The studies have also shown that, for a given fuel injection pressure and nozzle configuration (number of spray holes, spray hole effective area and direction of the fuel sprays) the axial spray/jet momentum depends on the efficiency of the injector and the nozzle in converting the upstream fuel injection pressure into axial momentum in the fuel spray/jet.
A test rig and technique for assessing the axial momentum in a fuel spray/jet is described by Desantes et al (Desantes J M, Payri R, Salvador F J and Gimeno J, “Measurements of Spray Momentum for the Study of Cavitation in Diesel Injection Nozzles”, SAE 2003-01-0703). The technique involves measuring the force exerted by a fuel spray/jet on a plate positioned perpendicular to the spray axis and arranged to destroy the axial momentum of the fuel spray/jet. In this technique, the fuel spray from a spray hole of a dynamically operating diesel injector is aimed perpendicularly towards the plate with a force transducer.
The technique described by Desantes et al. utilises a dynamically varying electrical output signal from the force transducer with each injection event to give a spray force versus time diagram for the fuel spray as shown in FIG. 1. The raw spray force signal from this technique tends to vary rapidly with time, owing partly to the turbulent structure of the impacting fuel spray and the vibration forces induced in the test rig from the dynamic operation of the injector. Electronic frequency filtering can be used to give a smoother force signal, but the absolute accuracy of measuring the spray force, and hence spray momentum, does not meet the standard required of modern nozzle designs which must be measurable to an accuracy of at least one percent.
A parameter known as the nozzle discharge coefficient Cd is defined in equation 1 below, and is commonly used to assess the actual (or measured) mass flow {dot over (m)} of fuel through an injector nozzle relative to a theoretical maximum mass flow {dot over (m)}t value.
                              C          d                =                              measured            ⁢                                                  ⁢            mass            ⁢                                                  ⁢            flow            ⁢                                                  ⁢                          (                              m                .                            )                                            theoretical            ⁢                                                  ⁢            mass            ⁢                                                  ⁢            flow            ⁢                                                  ⁢                          (                                                m                  .                                t                            )                                                          (        1        )            
The actual mass flow {dot over (m)} can be calculated using equation 2 below, by measuring the volume V of fuel flowing through an injector nozzle during a predetermined time period T.
                              m          .                =                              V            ·                          ρ              f                                T                                    (        2        )            where ρf is the fuel density.
The theoretical mass flow {dot over (m)}t is calculated according to equation 3 below, by using the theoretical Bernoulli velocity Ut of fuel emerging from a nozzle spray hole of geometrical area A (based on the minimum spray hole diameter along the spray hole length).{dot over (m)}t=ρf·A·Ut  (3)By combining equations 1 and 3, the nozzle discharge coefficient Cd is given by equation 4 below.
                              C          d                =                              m            .                                              ρ              f                        ·            A            ·                          U              t                                                          (        4        )            where Cd·A is the effective spray hole area relevant to the fuel mass flow.
Although the nozzle discharge coefficient Cd is useful when calibrating a fuel injection system to deliver the correct quantity of injected fuel, it does not directly quantify the momentum of the fuel spray, and hence the emissions performance of the nozzle. Further, it can be difficult to measure the nozzle diameter or spray hole area A, particularly with certain shapes of spray hole nozzle such as convergent spray holes which are often used in advanced nozzle designs.
It is important to be able to measure and monitor the sample-to-sample quality of production injector nozzles, and to assess rapidly the effectiveness of new nozzle design improvements at reducing emissions. The invention aims to define a new parameter which provides an indication of the engine exhaust emissions performance of the nozzle. The invention also aims to provide a method of measuring this new parameter to a sufficiently high degree of accuracy, with a relatively simple and cost-effective rig test.
It is known that the emissions performance of an injector nozzle is also sensitive to the effective direction of fuel spray from the injector nozzle. For example, it is known that emissions performance can be affected by the exact point of impact of the fuel spray with the wall of the combustion chamber in the engine piston. The direction of the fuel spray jet might be expected to coincide with the spray hole axis which in principle should be known from the design of the nozzle. In practice the target direction of the fuel spray may deviate slightly from the spray hole axis, for example with advanced nozzle designs which may have converging spray hole shapes or spray holes with inlet rounding.
The effective direction of the fuel spray is commonly measured using optical techniques. However, optical techniques can present problems because the edges of the fuel spray/jet are often indistinct, which makes measuring the direction of the fuel spray to a sufficient degree of accuracy difficult. The invention therefore aims to provide a test rig and associated technique for measuring accurately the direction of fuel spray from an injector nozzle.
Further, it is known that the emissions performance of an injector nozzle in an engine is sensitive to the initial included angle of the fuel spray/jet emerging from the nozzle spray hole. This is because the initial included angle determines how quickly the fuel spray/jet initially entrains the surrounding air charge in the engine combustion chamber. For example, if the initial included angle is too large, the fuel spray/jet will expand and entrain the air charge in the combustion chamber too rapidly, and hence decelerate too rapidly. The result of this is that the fuel spray/jet does not penetrate far enough into the combustion chamber. This under-penetration means that the fuel spray/jet does not utilise the air available in the outer radii of the combustion chamber, thereby causing higher exhaust emissions. Conversely, if the initial included angle is too small, the fuel spray/jet will penetrate too far into the combustion chamber before it expands in width sufficiently to entrain the air charge. This over-penetration may result in liquid fuel being deposited on the walls of the combustion chamber, thereby causing unburnt hydrocarbon emissions.
In common with the effective direction of the fuel spray described above, the initial included angle is also commonly measured using optical techniques. However, optical techniques also present problems for measuring the included angle to a sufficient degree of accuracy because of the indistinct edges of the fuel spray/jet. The invention therefore further aims to provide a test rig and a suitable technique for measuring the included angle of a fuel spray/jet to a sufficiently high degree of accuracy.